Tutorial was written by me so please give credit if your gonna go post it on another site. Also credit for all YouTube tutorials goes to their respected owners. If I helped you at all don't forget to say Thanks!
What you'll need :

A small wrench

Torx T8

Torx T10

A small flat head screwdriver.

Isopropyl (rubbing alcohol)

Thermal Paste

A heat gun

Brasso (Optional)

No-Clean Flux

Xclamp repair kit Recommended but again this is optional. You can pick one up on eBay dirt cheap. They often come with tutorials so I won't be going through that.

For the 12v fan mod (optional part):

Heat shrink tubing, you can use insulation tape though heat shrink tubing is better

Some wire. Old speaker wire is good but generally any wire that is able to carry 12v should be fine.

A lighter

A craft knife

How to re-flow your 360:

1. Take your motherboard out of the metal shell. Heres a nice tut to dismantle your 360:

2. Remove the xclamps. Use a small fat head screwdriver to pry them off gently.

3. Remove the heatsinks by using the wrench to un-tighten the bolts.

4. Clean off the old thermal paste from the GPU and CPU using a tooth pick to scrape it off.

5. (Optional)
I found that Brasso brings it off extremely easy but it is conductive so you've got to be careful with it. A lot of people would reccomend not using it but I've had no trouble at all with it.

6. Clean the CPU and GPU with Isopropyl. This will clean off the Brasso if you used it, if not still use the isopropyl as it does help to clean it.

7. When done the CPU and GPU should be nice and shiny.

8. Now there is a small gap between the motherboard and the CPU and GPU, apply some no-clean flux between that gap whilst holding the motherboard upright. You should see it running out of the other side if done properly. Do this with all four sides of the chips.

9. Once done take the metal shell the motherboard sits in and place your motherboard face down on top of it.

10. Take your heat gun and go over the entire bottom side back and forth at about a fists length away for 1 minute.

11. Take your motherboard and flip it over. Heatgun the CPU, GPU, RAM, Southbridge and HANA in a circular motion for 4 minutes. Avoid any direct contact with any of the capacitors and plastic parts. If you don't know what the CPU, GPU, RAM, Southbridge and HANA are take a look at the video in the link and it will show you which parts you'll need to go over.

12. When you've done that leave the board for 30-45 Minutes. In this time DO NOT touch the board or knock it at all.

13. After you've waited 30-45 Minutes take your mother board and apply a small drop of thermal paste to the CPU and GPU dont overdo it with the thermal paste a drop about the size of a grain of rice is enough. The GPU has two seperate little chips so apply it to both.

14. Replace your heatsinks and the bolts and washers if your using an xclamp repair kit (don't over tighten the bolts). If you want to replace your old xclamps go ahead and do so but I would recommend the xclamp repair kit to stop the motherboard warping.

15. Re-assemble your 360.

16. Plug in the PSU and AV cable.

17. Turn your 360 on and VIOLA! You should see a picture now on the tv/ no more RROD.

Recommended but optional, 12v Fan mod:

This will keep your 360 running cooler preventing problems in the future. The stock fan is a 12v fan but M$ set it to use only 5v then build up speed as the console got hotter which by the time the console is hot its too late. This will make your fan run at 12v all the time. Be warned though your xbox will be quite noisy but if you play with your TV a little louder then you shouldn't have a problem.